A wood bark manufacturing method is known from international patent publication WO 93/25324. According to this, the wood chips with bark attached are ground initially either with a special plate grinder, or with a vibrating cone crusher, wherein the bark separates from the wood chips and the size of the bark particles diminishes. Thereby, the bark can be separated more easily during later separation stages. A grinder or a vibrating cone crusher is followed by pneumatic separation, to separate outer birch bark, after which sawdust is removed using a vibrating sieve. The bark content has now dropped to considerably less than 10%, and the final cleaning can be carried out using an optical sorter. According to the patent, two optical sorters are used in series, but can, however, be replaced by a single more powerful device. The accepted fraction from the second optical sorter is led to grinding and the reject to the fuel fraction.
Equipment that is essentially that described by the application has been built in Kankaanpaa in Finland, the supplier being BMH Wood Technology Oy. In the commercial equipment, a magnetic separator and a pneumatic separator are used in the pre-cleaning to separate metal scrap and stones. Sawdust is removed from the commercial equipment before grinding, and after grinding the vibrating sieve is replaced by a so-called pocket-roll sieve (a roller sieve developed especially for sieving sawdust). In the commercial equipment, a good yield is achieved with a bark content of about 1%, which is sufficiently clean for the manufacture of certain grades of cellulose. The yield varies between 60-70%, depending on the species of timber and other factors. If cleaner chips are wanted, the yield drops, and correspondingly improves with a poorer level of cleanliness.